Friday, 3 February 2012

Entangled diamonds blur quantum-classical divide

Two diamonds as wide as earring studs have been made to share the spooky quantum state known as entanglement. The feat, performed at room temperature, blurs the divide between the classical and quantum worlds, since typically the quantum link has been made with much smaller particles at low temperatures.

Entanglement is one of the weird aspects of quantum mechanics, where the fates of two or more particles are intertwined – even when they are physically far apart. Electrons, for example, have been entangled, so that changing the quantum spin of one affects the spins of its entangled partners.

Macroscopic objects, on the other hand, are supposed to mind their own business – flipping one coin shouldn't force a neighbouring flipped coin to come up heads.

But that's just what happened with two 3-millimetre-wide diamonds on a lab bench at the University of Oxford. Physicists there led by Ka Chung Lee and Michael Sprague were able to show that the diamonds shared one vibrational state between them.

Other researchers had previously shown quantum effects in a supercooled 0.06-millimetre-long strip of metal, which was set in a state where it was vibrating and not vibrating at the same time. But quantum effects are fragile. The more atoms an object contains, the more they jostle each other about, destroying the delicate links of entanglement.

Fleeting link

Cooling an object down to fractions of a degree above absolute zero was thought to be the only way to keep atoms from doing violence to each other.

"In our case we said, let's not bother doing that," says Ian Walmsley of Oxford, head of the lab where the diamonds were entangled. "It turns out all you need to do is look on a very short timescale, before all that jostling and mugging around has a chance to destroy the coherence."

The team placed two diamonds in front of an ultrafast laser, which zapped them with a pulse of light that lasted 100 femtoseconds (or 10-13 seconds).

Every so often, according to the classical physics that describes large objects, one of those photons should set the atoms in one of the diamonds vibrating. That vibration saps some energy from the photon. The less energetic photon would then move on to a detector, and each diamond would be left either vibrating or not vibrating.

But if the diamonds behaved as quantum mechanical objects, they would share one vibrational mode between them. It would be as if both diamonds were both vibrating and not vibrating at the same time. "Quantum mechanics says it's not either/or, it's both/and," Walmsley says. "It's that both/and we've been trying to prove."

Same state

To show that the diamonds were truly entangled, the researchers hit them with a second laser pulse just 350 femtoseconds after the first. The second pulse picked up the energy the first pulse left behind, and reached the detector as an extra-energetic photon.

If the system were classical, the second photon should pick up extra energy only half the time – only if it happened to hit the diamond where the energy was deposited in the first place. But in 200 trillion trials, the team found that the second photon picked up extra energy every time. That means the energy was not localised in one diamond or the other, but that they shared the same vibrational state.

Entangled diamonds could some day find uses in quantum computers, which could use entanglement to carry out many calculations at once.

"To actually realise such a device is still a way off in the future, but conceptually that's feasible," Walmsley says. He notes that the diamonds were entangled for only 7000 femtoseconds, which is not long enough for practical applications.

Quantum limit

The real value of the experiment may be in probing the boundary between quantum mechanics and classical physics. "We think that it is the first time that a room-temperature, solid-state system has been demonstrably put in this entangled quantum state," Walmsley says. "This is an interesting avenue for thinking about how quantum mechanics can emerge into the classical world."

"We want to push and see how far quantum mechanics goes," she says. "The reported work is a major step in trying to push quantum mechanics to its limits, in the sense of showing that larger and larger physical systems can behave according to the 'strange' predictions of quantum mechanics."

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